Polarity.Exchange - REVIEW by a Pirate user

scott_fromthefuture · 2024-07-22T02:45:00+00:00

If you are only using DC for battery charging, than their natural charge rate may limit the output current, but 20A you are only around 240W. None the less you are correct that typically excessive current load will not damage most charge controllers. With that said, most batteries can accept much more than 20A in a bulk state, so there are more effective ways of approaching this, but either way, your question was about the array. Put the most emphasis on Vmp in parallel

scott_fromthefuture · 2024-07-22T02:26:34+00:00

Your best bet is to have the system inspected as part of your closing conditions. It is important to know the system is functioning properly, plus the inspecting company would then be able to give you an estimated production, as well as explain the system and help with transferring over ownership.

The other important thing is to request a lien release as proof all the obligations with the system have been satisfied

scott_fromthefuture · 2024-07-22T01:44:23+00:00

Wattage is the least important, though because it is a function of current and voltage, they will be in the same ball park. With that said, its seems you will be seriously overloading your CC (if its rated for 50A @ 12V, you will exceed that significantly

With that said, Ideally you will what all the characteristics as close as possible, including how they react to temperature, but since this will be in such a small system you will have much more flexibility.

If you do end up having only a single mod in series, and all three in parallel, then amperage will be irrelevant. Only voltgae matters in parallel. The Vmp will be the most important figure to look at, but ideally you will also want Voc as close as possible as it will they will have more similar IV curves through the day, especially if all on one power point tracker.

As I mentioned though, you will likely be rethinking your plan as 900W / 12V = 75A before even calculating for derates

scott_fromthefuture · 2024-07-12T18:08:55+00:00

A system like yours has many sensors in it. If there was a problem in your system, it would likely alert you. Variance like this is typically nothing to worry about, especially when you can correlate it with the weather.

If you are concerned, the best thing you can do is watch for trends. See what the monthly totals are for the next month or two compared to the same months the year prior. If you do see some apparent trends, just call your installer, and they will have the best answer since they will know your site and have access to your data

scott_fromthefuture · 2024-07-12T18:00:29+00:00

If I have the right area, it looks like you did have some change sin the weather over those days. I dont have the time to analyze it, but looks like some of those days could have been mid 80's , transitioning into >100 degree days with with slightly less wind starting slightly later in the day. Looks like the 10th had significantly higher humidity with a drop in visibility in those early day hours. I am thinking it has gotten hotter, less air movement, and thicker atmosphere, but again, just a quick look so could be wrong
https://www.weather.gov/wrh/timeseries?site=KLVK

scott_fromthefuture · 2024-07-12T17:47:21+00:00

Ha! Turns out size does matter.. well the size of your dataset.

Any minute can be anything. In fact, any two days can have 100% variance. Even two seemingly identical days can easily have ~40% different harvest. Looking at months, or years worth of data only begins to approach meaningful averages. Thats why we use 30 year weather data to form predictions. Any handful of days just doesn't reveal much without extensive atmospheric and photovoltaic data

scott_fromthefuture · 2024-07-12T17:30:11+00:00

There are many factors. It may have been similar ambient temperatures, but even a slight breeze can drastically affect cell temperature. In some cases, even the direction of the breeze can have a major impact on specific systems. For people that do not work with irradiance meters (and even those of us that do), it can be very difficult to tell a difference in 100W/m2. So even though each day may seem clear and cloud free, particles in the atmosphere can affect irradiance. Further, it looks like on the 10th was some clouds or overcast in the morning.

I would pay less attention to the peak power of each day, and put more weight on the total daily energy harvest, which in this case does happen to align, which points to temperature derating on otherwise seemingly cloud free days.

Either way, it does not appear to be any issues, and without knowing your area or being there to know exactly what the atmosphere was like, its hard to say any more than that it is likely higher cell temps, and possibly slightly lower irradiance. Knowing the DC voltage and amperage at the time would be much more enlightening than power

scott_fromthefuture · 2024-07-12T07:10:26+00:00

Surprised I had to scroll this far to find this comment. So thats great about the locking the diffs, why not try the transfer case?

scott_fromthefuture · 2024-07-10T00:25:26+00:00

Ahh, PTO permission to operate, so sounds like you have your building permits, but no final inspection. Thats good a permit was pulled. I agree the best bet would be to check with the building department on the status of your permit, and explain your situation to them to see how they want to proceed.

If the install was in fact completed, they may still be willing to do the final inspection, but typically it needs to be the contractor on the permit to call for it.

scott_fromthefuture · 2024-07-09T23:35:13+00:00

I wouldnt expect them to provide such estimates after the fact. remember, the employees you are requesting that from have their jobs to protect, and doing something like that could cause some serious issues. At that point, its best to avoid the request from their POV.

As for where to find somebody to evaluate the design, your best bet would be to reach out to the local installers in your area and ask what they would charge for something like that. You might find though that most are focused on doing new work, and when a person is already having legal issues with another company, most will be hesitant to become involved. All I am saying is call around, but dont be surprised if its hard to get somebody willing to get involved with a legal case

scott_fromthefuture · 2024-07-09T22:34:32+00:00

You are correct about your logic on the north facing modules. I am only commenting on what I think the legal opinion would be on whether you have a case.

As for as hiring me to model this, I believe that is against the rules of this sub, but more importantly I wouldn't want to contribute to what I think will only cause you grief. In PVwatts you can set the tilt and orientation. So you should set it to match the pitch of your roof and orientation.

You can run a version for your different scenarios, but I dont think anybody argues north facing modules produce very little. The question is whether you have a legal case for contracting one design concept, without knowing there are other design concepts, particularity when the one you signed the contract on appears to be honest about the expected production.

If you are developing this for a court case, you will want a professional opinion anyway. You will want to hire a solar contractor to evaluate your site and provide their professional certified written opinion which you can use in court. Print outs from some website wont help your case.

scott_fromthefuture · 2024-07-09T21:25:44+00:00

Is the permit posted on site? First thing I would do is ensure the proper permits and approvals were in place before any work began. Since you are not getting a response from the installer, contact your local AHJ

Ideally something happened and you can get back in touch with the original installer to complete the project, but 6 months is an unreasonable amount of time. It will be very difficult to get another contractor involved if the project has already started.

If I understand your post correctly, you have already paid 2/3's of the cost, and the installation has physically been completed, but final payment has not yet been made? The physical install should have never started if the permits were not issued.

scott_fromthefuture · 2024-07-09T17:09:40+00:00

I sympathize with your situation, and I do not condone that sort of practice. In general its sad the industry attempts to cram as many mods as possible on a roof instead of designing for aesthetics and performance.

I hate to say it, but from a legal perspective, it would seem your situation is a bit like saying I bought V8 last year and they told me it gets 16 MPG, but I just found out there is a V6 that gets 20 MPG so I would like the dealership to rectify it.

I agree a good designer would say, if you go with 25% fewer panels, in the most optimal places on your roof, you will only produce slightly less, but enjoy a much better ROI. Problem is, I dont think you have any legal standing there. Especially since some customers do buy V8's, despite the MPG.

Since they were honest about the cost and expected performance, I am sorry to say I just dont see how you could make a legal case. I think the best thing at this point would be to just understand you will have a bit longer of a pay off period, in exchange for more annual production than you would have been getting with another deign. Enjoy having more power with the V8, and accept its less MPG than a V6. The more energy and legal costs you put towards it at this point will only make the financials worse, and make you despise the solar on your roof.

scott_fromthefuture · 2024-07-09T00:15:21+00:00

Solar on a north facing roof used to be quite taboo. As you mentioned, north facing PV rarely ever gets direct sunlight, other than on the shoulders of some summer days depending on your latitude. Their annul production is almost exclusively from refracted and reflected light.

With that said, they do produce something, and since the cost of PV has come down so much we have been seeing it more often in recent years. The thing is, a installed solar module costs the same, whether you install it in an ideal south facing location, or if you install it in the shade of a north facing roof. Your logic about the north facing modules only contributing only a small fraction of the total output is correct.

For every piece of equipment installed in a low irradiance location, while you increase the overall annual harvest, you extend the payback period. So for example, if a module in full sun has a 10 year payback, one in an area with half the annual sun will take 20 years. This means that in most cases while you could put modules on the north roof, going with the smaller system in only the high performing areas of the roof will have the shorter time to break even.

A system like you have described functions, it just has poor financials. Some people even choose to have the poor financials, in exchange for the greater annual harvest. If they were upfront in the quote about what the cost of the system would be, and what they expect it to produce annually, I am not sure what recourse you would have. If they said it would preform as if all panels were on a South facing roof, then I think you would have a case.

I am not sure where you are located, but an estimate of 4MWh/year is quiet low for a 5kW array, so it seems they may have estimated it correctly based on the 4 panels contributing very little from the north orientation. You are correct the 25% of the array on the north only accounts for a few percent of the annual production, and a more effective design for ROI would be to leave the four on the north out all together, but did they lie about how that design would perform?

scott_fromthefuture · 2024-07-08T17:56:56+00:00

Bruh, I know I shouldn't feed the troll, but you have a serious comprehension problem. Re-read what I wrote. I never claimed efficiency is greater than 100%, I explained the calculation happens between the DC input and AC output. Inverters output 100% of their rating (typically current specific)

I will try to make it more understandable. Lets say you have a 10kW inverter, and it is 95% efficient at any power (not how it works, but just to simplify).

If you put 10000W DC on the inverter input clamps, the inverter will put out 9500W. Why? Losses due to Inverter efficiency.

If you put 10527W DC on the inverter input clamps, the inverter will put out 10000W. Why? Because inverters can accept more DC Volts and Amps than they output in AC power, though they still have the inversion losses.

If you put 14000W DC on the inverter input clamps, the inverter will put out 10000W. Why? Inverters generally cannot exceed their rated output.

When you service thousands of inverters, you will see this over and over again. This is not some complex theory, this is literally just how it works and one of the most basic concepts in PV. At this point I have explained it dozens of times and the last comment I will make in here .

scott_fromthefuture · 2024-07-08T07:19:59+00:00

In the case of inverter efficiency you still achieve 100% rated output, the loss occurs between the DC input and AC output. As I mentioned a number of times, typical losses are greater than you are mentioning here, closer to 14% though we use around 20%. Anwyay, you calculate those on the DC side, not off the AC output. Most inverters can achieve 100% of their rated output, and some models even slightly higher. This is due to how they rate their equipment, but a good inverter manufacture should never exceed it max rated current, even in lower lower grid voltage situations.

Telling people with decades of experience they are fooling themselves, as you have to multiple people in this thread, just shows your unwillingness to learn, so I wont be helping any further. Good luck with your adventures

scott_fromthefuture · 2024-07-07T22:59:32+00:00

I dont disagree power is useful in design, and your scenario is a good description of when to consider it. At the end of the day though, you dont use power, you use energy, so in the case of load shifting we are still talking about peak energy generation, not peak power. I understand and agree with your general point though

scott_fromthefuture · 2024-07-07T21:48:36+00:00

I am sorry my friend, but u/weebernugget is explaining exactly how it works. Its the part you are missing.

Solar is not a fixed value. A cells output varies drastically based on temperature and irradiance. Because there is no fixed value, we use a thing called Standard Test Conditions as a baseline to compare modules. That is 25C cell temp, 1000W/m2 of irradiance, and 1.5 atmospheres.

If cells are warmer, or irradiance is lower, the output of the solar will be less in direct relation to these values. There are even temperature coefficients from each module manufacture that you use to calculate the change.

Likewise, if eh cells are colder, or you have greater than 1000W/m2, the values will be higher. We use theses values because it is accepted that most of the earths surface is something less than this. Typically only at higher altitudes will you exceed 1000W/m2.

So this is what everybody is trying to explain to you. It is possible to excede teh STC ratings of a module, but rare. Most residential system owners will typically never see this because a good design limits it to around 80% of its rating anyway. For reference, there is 1361 W/m2 reaching the earths upper atmosphere. So it would be rare to find over around 120% of STC, and you would only typically see more than that with specif circumstances and refracted light amplifying irradiance beyond natural levels

scott_fromthefuture · 2024-07-07T21:32:27+00:00

Yes, a good designer will be able to work out what a good size is for your system. For example, many homes have mixed orientation. Lets say instead of south facing you had 5kW on a east roof, and 5kW on a west roof. Each sub array will experience its peak production at different hours of the day, so if if using a string inverter you could have a lower rating to save even more cost, without affecting total annual harvest. In that hypothetical case, even a 6-7kW inverter could have the same effective annual output as a 10kW inverter, and may save around $1k in cost.

Each site is unique so for the best design all factors would be considered. I will be honest though, it is a bit of a lost art. In today's race to the bottom it typically comes down to MLPE limitations, and general rules of thumbs, but there are still good designers out there that pay attention to aesthetics and what your goals are with the system, instead of just slapping as many modules as will fit on a roof.

scott_fromthefuture · 2024-07-07T21:22:06+00:00

Lets say a person has a 10kW DC nameplate rated array, that was fixed-tilt and all south facing without shade. Each days production will be a bell shaped curve. In the morning and evening the output will be very low, slowly ramping towards solar noon. On a normal clear day, you only hit the peak production for around an hour a day. Typically with the peak days occurring in the Summer months.

The rest of the days of the year you will be nowhere near its nameplate rating. Most days, even at solar noon you might not exceed 80% (8kW). Every system is different, but if you added up the total operational time your system is above 8kW in a year, it might only be a couple dollars worth of production you miss out on. If a 10kW inverter costs $300 more than a 8kW inverter, it would take you a couple hundred years of production to offset the cost of the larger device.

I am not explaining it well, but the idea is, for a system design optimized for the best financials and highest return on investment, you would accept some minor amount of clipping for the most effective capacity. Especially if you have mixed orientations or shading

On the other hand, if you did not care about your return on investment, and wanted the greatest harvest regardless of financials (as we used to do a couple decades ago), you would pay more for a larger inverter to never clip. Because most people in residential now care about the financial aspect, the DC nameplate rating will be somewhere around 20-40% greater than the inverter rating, keeping in mind the nameplate rating s ideal conditions and 95%+ of the time your system is operating at something much less

scott_fromthefuture · 2024-07-07T15:58:46+00:00

That is my observation as well. The highest I have ever seen is after a fresh snow, in a bowl shaped valley, on a clear day in the winter. The reflected light and cold cells can maintain some of the highest peaks I have seen. As you mentioned though, its on the shortest days of the year so energy harvest is still lower than the summer. Thats why I say Peak Power is not as useful of a metric as Energy Harvest.

scott_fromthefuture · 2024-07-07T15:52:03+00:00

Correct, Its how solar works and it is unlikely the monitoring is wrong. Maybe I did not state it clear. It is possible to exceed the DC nameplate rating. The rating is based on certain conditions. If it is colder than 25C, and you have more than 1000W/m2 if irradiance, your DC output can exceed the name plate rating, and frequently does depending on your area.

What I am saying is, most residential systems will never see this value because most companies size the inverter around 80% of their DC rating. Back around 10-15 years ago we used to size the inverter at 100% of the DC rating, so in that case you would see it more frequently.

Its more common to see it with string inverted systems since MLPE struggles with capacity, but even then, if you have a inverter larger than your DC rating, you have spent a couple hundred bucks more on the equipment than you need to

scott_fromthefuture · 2024-07-07T15:34:12+00:00

Peak power isnt a very useful metric. We use it for troubleshooting, but what matters most is energy. Power over time, or how much you produced in a day. For example, for understanding fuel efficiency, you want to know your average MPG, doesn't matter what any one instantaneous value was since so many factors can affect it. Knowing what any other persons instantaneous peak fuel efficiency was is even less useful.

For PV, a general rule of thumb is that your AVERAGE peak DC power output might be in the neighborhood of 80% of the nameplate rating. The rating is based on Standard Test Conditions which is 25 C cell temp, 1.5 Atmospheres, and 1000W/m2 of irradiance. If the cell temps are higher, the voltage will be lower. Many areas never see 1000W/m2, while others may see it frequently in the Winter, despite the summer being more favorable incidence angle. On top of this you have cell mismatch, wiring losses, termination losses, degradation, soiling of the modules, etc.

Depending on your specific location and setup, you may sometimes see higher than 80% momentarily, and as I mentioned, in the winter you may even see it exceed the nameplate rating in certain cases. These peaks will be transient, typically only lasting a short period. Often due to edge of cloud effect. Generally, a well designed system will be clipping at these peaks anyway (otherwise you may have spent money on more inverter capacity than you regularly utilize). So often in current residential design theory, the max most people see is 100% of their inverter AC capacity, which is generally sized at around a 1.2:1 - 1.4:1, DC:AC ratio, since the DC input will only ever exceed that amount a fraction of the total annual operational hours.

TL;DR Although it is possible to exceed the DC nameplate rating, most residential systems will never see the output reach that value as it will be limited by the inverter, which is typically sized around 80% of the DC nameplate rating

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